Evidence for eosinophil involvement in various human diseases is increasing. The eosinophil is not a bystander cell, but rather an active participant in allergic and other inflammatory diseases. However, the mechanism of inflammatory mediator release from eosinophils at the inflammation sites, and the significance of eosinophil-derived mediators for disease pathophysiology are not clear. During the current funding period, our observations suggest that eosinophil degranulation involves antigen-specific IgG antibodies, but not IgE antibodies. Further, the attachment of the cells to their targets through adhesion molecules and the strong amplification of signals by endogenous platelet-activating factor (PAF) produced within the cells are complex phenomena. This application uses which knowledge and seeks to dissect critically the mechanisms and significance of eosinophil degranulation in human diseases. First, we will analyze the "molecular mechanisms of eosinophil degranulation" in vitro. Specifically, we will address how PAR, in controls to other chemotactic factors, strongly stimulates release of inflammatory mediators from eosinophils. We hypothesize that eosinophils have intracellular as well as extracellular PAF receptors and that the degranulation response to PAR uses distinct receptor/signaling mechanisms from those used in chemotaxis. Second, we will examine the roles of antigen-specific IgG antibodies as "triggers of eosinophil degranulation" at the inflammation sites. This aim combines in vitro experiments to examine the roles of the complement system in eosinophil activation with analyzes of airway and skin disease specimens to investigate the presence of eosinophil-activating IgG antibodies. Subsequently, the roles of IgG antibodies in the pathophysiology of eosinophilic inflammation will be examine directly in IL-5 transgenic mice. Third, the observations in the first and second aims will be culminated in vivo studies examining the "significance of eosinophil degranulation" in diseases using a mouse model of asthma. We will examine the following question. Is eosinophil major basic protein involved in the development of airway hyperreactivity? Does the induction of inhibition of eosinophil degranulation in the airway modulate airway physiology in these animals? The results of these experiments will provide a better understanding of the mechanisms of eosinophil degranulation in vitro and in vivo and clarify the roles of eosinophils and their granule proteins in the pathophysiology of human diseases.